The present invention relates to a high frequency transmission line for transmitting a high frequency signal, a resonator having the high frequency transmission line, and a high frequency device having the resonator.
This invention deals with intermodulation distortion which refers to the family of system performance impairments caused by the nonlinear transfer characteristic of a broadband system, which produces spurious output signals (called "intermodulation products") at frequencies that are linear combinations of those of the input. The system output (S.sub.0) can generally be related to the system input by the transfer equation: EQU S.sub.0 =AS.sub.t +BS.sub.t.sup.2 +CS.sub.t.sup.3 AS.sub.t =the fundamental signal term
The terms BS.sub.t.sup.2 and BS.sub.t.sup.2 represent the second order and third-order distortion terms, respectively. As shown in FIG. 3, the second order term produces a second harmonic frequency component for every input signal frequency and intermodulation frequency components of the form f.sub.1 .+-.f.sub.2. The third-order term produces a third harmonic frequency and intermodulation frequency components of the form f.sub.1 .+-.f.sub.2 .+-.f.sub.3 and 2f.sub.1 .+-.f.sub.2. Third-order distortion also produces cross-modulation where modulation of one carrier can appear on another carrier on the system even when the second carrier is unmodulated when input into the system.
For convenience of expression, the term "twin walls" is used herein to describe a superconductor layer made of LnBa.sub.2 Cu.sub.3 O.sub.x crystals. More particularly, a non-grain boundary oxide superconductor layer can be made of LnBa.sub.2 Cu.sub.3 O.sub.x crystals having an orthorhombic system in the crystal structure. A rectangular parallelepiped crystal has lattice spaces a, b and c extending in x, y, and z directions, respectively. The lattice spaces a, b, and c are not equal to one another in the orthorhombic system. It will be assumed as a particular condition that two LnBa.sub.2 Cu.sub.3 O.sub.x crystals are in contact with each other and are coincident with each other in the z direction while they are alternated with each other in the x and the y directions. In this particular condition, the crystals have a twin boundary therebetween, as is known in the art. Therefore, the contact is called "twin walls" throughout the following specification. The term "twin boundary" could also be used to describe the non-grain-boundary.
Recently, the mobile communication field is one of the fast-growing industrial fields where users have been doubled year by year. Following this, there have been such demands in the mobile communication field that as many users as possible can receive services under the excellent communication conditions without interference in the limited range of frequency allocation. For achieving this, in the mobile communication system using microwaves of, for example, about 0.8 GHz to 3 GHz, it is required that characteristics of resonators and filters used in base stations of the system be excellent. However, it has been difficult to satisfy such requirements by the conventional resonators or filters using gold or copper for conductor lines
Under these circumstances, attention has been directed to an adoption of superconductivity techniques. For example, there has been proposed a technique (hereinafter referred to as "prior art 1") which applies a thin or thick film made of RBa2Cu3Ox (R represents Y or one other element of the lanthanide series element, such as Nd, and x represents an amount of oxygen. Hereinafter, this compound will be referred to as "Y-123 type crystal oxide") having a 123 type crystal structure as being an oxide superconductor, to a conductor line in a high frequency transmission line in the form of a plane circuit having a microstrip structure, a coplanar structure, a stripline structure or the like.
It is considered that when the Y-123 type crystal oxide is used for the conductor line in the high frequency device according to the prior art 1, the characteristic of the foregoing resonator or filter can be improved as compared with using gold or copper. This is because the oxide superconductor is small in high frequency resistance.
In the prior art 1, a thin film made of Y-123 type crystal oxide and applied to a conductor line is formed to be about 0.5 .mu.m thick by a method of laser ablation or sputtering. On the other hand, a thick film made of Y-123 type crystal oxide is formed by a method in which a material dissolved in an organic solvent is hardened on a substrate. According to this method, a thick film not less than about 10 .mu.m in thickness can be easily formed.
On the other hand, there has been a problem about an intermodulation distortion characteristic, as one of high frequency characteristics, in the resonator or the filter which is applied with the thin or thick film of Y-123 type crystal oxide formed according to the prior art 1.
It is well known that when the intermodulation distortion is increased in the high frequency devices, such as the resonators or the filters, interference is, in general, immediately caused in the communication system using those high frequency devices. As described above, the foregoing technique is applied to the resonators or the filters for the purpose of allowing as many users as possible to receive the services under the excellent communication conditions without interference in the limited range of frequency allocation. Therefore, it is necessary to suppress the intermodulation distortion in the high frequency device to be small, that is, the intermodulation distortion characteristic should be improved.
In view of the foregoing, there has been proposed a technique (hereinafter referred to as "prior art 2") which aims to improve the intermodulation distortion characteristic by controlling the size of grain boundaries in a thin or thick film made of Y-123 type crystal oxide. As appreciated, the intermodulation distortion represents a non-linear distortion where an output signal including frequency components equal to the sums and differences between integer-times input frequency components appears, and the intermodulation distortion characteristic represents a characteristic thereof.
However, the high frequency transmission line formed according to the prior art 2 has the following problem upon being applied to the resonator or the filter as the high frequency device:.
Specifically, as described above, a lot of grain boundaries are included in the thin or thick film of Y-123 type crystal oxide formed according to the prior art 2. As a result, the surface of the thin or thick film is degraded in flatness to have irregularities thereon. This means that a corresponding transmission line is extended. Thus, the resistance is increased to enlarge a transmission loss particularly when the large power is inputted.
Further, although some evaluation results about the high frequency resistance and the high power characteristic according to the prior art 2 have been announced, the reliable experimental data have not yet been obtained so that reduction to practical use is still not possible.
Accordingly, since even the prior art 2, not to mention the other techniques, is not applicable, there have been no techniques available which can improve the intermodulation distortion characteristic in the high frequency device to suppress the harmonic distortion.